lysosome

A membrane-bounded organelle, found in
the cytoplasm of eukaryotic
cells, which contains digestive enzymes.
It acts as the "garbage disposal" of the cell by breaking down cell components that are no longer needed as well as molecules
or even bacteria that are ingested by the
cell. The interior of a lysosome is strongly acidic, and its enzymes are
active at an acid pH. Lysosomes are found in
all eukaryotic cells, but are most numerous in disease-fighting cells, such
as leukocytes (white blood cells).

Some human diseases are caused by lysosome enzyme disorders. Tay-Sachs disease,
for example, is caused by a genetic defect that prevents the formation of
an essential enzyme that breaks down ganglioside lipids.
An accumulation of undigested ganglioside damages the nervous system, causing
mental retardation and death in early childhood.

Details
of function and structure

Lysosomes break down cellular waste products, fats, carbohydrates, proteins,
and other macromolecules into simple compounds, which are then returned
to the cytoplasm as new cell-building materials. To accomplish the tasks
associated with digestion, the lysosomes
use some 40 different types of hydrolytic enzymes, all of which are manufactured
in the endoplasmic reticulum and modified in the Golgi apparatus.
Lysosomes are often budded from the membrane of the Golgi apparatus, but
in some cases they develop gradually from late endosomes, which are vesicles
that carry materials brought into the cell by a process known as endocytosis.

Like other microbodies, lysosomes are
spherical organelles contained by a single layer membrane, though their
size and shape varies to some extent. This membrane protects the rest of
the cell from the digestive enzymes contained in the lysosomes, which would
otherwise cause significant damage. The cell is further safeguarded from
exposure to the biochemical catalysts present in lysosomes by their dependency
on an acidic environment. With an average pH of about 4.8, the lysosomal
matrix is favorable for enzymatic activity, but the neutral environment
of the cytosol renders most of the digestive
enzymes inoperative, so even if a lysosome is ruptured, the cell as a whole
may remain uninjured. The acidity of the lysosome is maintained with the
help of hydrogen ion pumps, and the organelle avoids self-digestion by glucosylation
of inner membrane proteins to prevent their degradation.